This invention relates to chemical processing of normal (straight chain) alkane hydrocarbons and is more particularly concerned with a method of producing chlorinated normal alkanes at high terminal selectivity.
Terminally chlorinated normal alkanes, also known as primary chloroalkanes, are especially useful ingredients for the synthesis of numerous organic compounds. Such compounds include various fatty acids, amines, alcohols, esters, and sulfonates--which are in turn useful in the preparation of a wide range of commercial products.
As is well known, primary chloroalkanes are readily produced by direct reaction of normal alkane hydrocarbons with chlorinating agents, such as chlorine gas. The replacement of hydrogen of the alkane with chlorine may be facilitated by the presence of a reaction initiating agent such as light (photo initiation), heat (thermal initiation), or a catalyst such as a peroxy compound (chemical initiation). Such reactions are generally not used in the preparation of primary chloroalkanes in pure form, however, because they produce not only the terminally substituted species, but a mixture of reaction products from which the primary product must be separated. To compound the problem, these reactions substantially favor the formation of secondary chloroalkanes--that is, chlorination of interior carbons of the alkane chain rather than of the terminal carbons. The higher selectivity for secondary chlorination is due to the greater number of secondary hydrogens (in long chain alkanes) and their greater reactivity in comparison to the terminal or primary hydrogens. Chlorination of a normal alkane thus generally results in a relatively large proportion of secondary product and only a small proportion of primary product. While the two products can be separated, the yield of primary product is ordinarily too small to justify the effort. Accordingly, the art has been forced to use other techniques of producing primary chloroalkanes.
In an effort to make direct chlorination a commercially feasible alternative for primary chloroalkane preparation, researchers have sought ways to control the reaction in such a manner as to increase terminal selectivity. Terminal selectivity is commonly defined in either of two ways: as the weight percent of terminally chlorinated product in the total monochlorinated products produced by the reaction (denoted herein as "S.sub.t "), or as the probability of reaction of a primary hydrogen versus a secondary hydrogen for the monochlorinated products (denoted herein by "S.sub.o ").
Ideally speaking, a controlled reaction should maximize not only terminal selectivity, but also conversion and selectivity for monochlorination. Conversion is defined as the weight percent of alkane consumed in the chlorination reaction. Selectivity for monochlorination (denoted herein by "S.sub.m ") is defined as the weight percent of monochlorinated product in the total chlorination products. Thus, for a given terminal selectivity, it will be apparent that the higher the conversion and selectivity for monochlorination, the higher will be the yield of primary product.
One proposed technique for increasing terminal selectivity of the chlorination reaction involves preadsorption of the alkane with a particulate adsorbent. In an article entitled "Directing a Chlorination Reaction," which appeared in the Journal of Organic Chemistry, Vol 35, No. 6, 1970 (pages 2053-2054), Eli Perry reported that increased selectivity for 1-chlorohexane could be obtained by chlorinating normal hexane adsorbed into an X type zeolite. X type zeolites are low silica zeolites (silicon-to-aluminum ratio around 2-3 by weight) characterized internally by a network of chambers interconnected by channels. Perry's best values for S.sub.t (about 50%) and S.sub.o (about 3) held for conversions of only about 1%. At 2% conversion, terminal selectivity dropped significantly; and at commercially practical conversion levels, terminal selectivity was not much better than for neat reactions (S.sub.o about 0.4, S.sub.t about 15%). Perry was unable to attain good product recovery levels with channel sizes less than 10 angstroms.
In U.S. Pat. No. 3,951,770, David McCoy reported improved terminal selectivities at higher conversions with non-zeolitic (non-porous) adsorbents. McCoy achieved terminal selectivities as high as about S.sub.t =27%, S.sub.o =1.2 for 1-chlorododecane, but results ranged more typically around S.sub.t =15%-20%, S.sub.o =0.6-0.8, depending on the adsorbent. As a basis for comparison, McCoy ran tests with certain A type and mordenite zeolites. These zeolites failed to give any appreciable improvement over neat (no adsorbent) chlorination. As will be of apparent significance later herein, the A type zeolites have a similar internal structure to the X type zeolites and a slightly lower silicon-to-aluminum ratio (about 1 by weight); and the mordenites have a unidirectional internal channel network with no chambers.